Prior Art
U.S. Pat. No. 3,824,002 in the name of Terry D. Beard, entitled "Alternating Current Liquid Crystal Light Valve" and assigned to the same assignee as the present invention teaches the basic principles of operation of an alternating current liquid crystal light valve which requires that a photoconductor be impedance-matched to the liquid crystal, the photocapacitance of the photoconductor being modulated in response to an input light.
In one preferred embodiment described in that patent, cadmium sulfide was thermally deposited on a heated substrate and subsequently baked in a hydrogen sulfide atmosphere to bring about a more nearly stoichiometric film whereupon cadmium telluride film was deposited in a vacuum onto the cadmium sulfide. In the embodiment disclosed by Beard, the cadmium sulfide film was about 2 to 12 microns thick and the cadmium telluride film was about 2 microns thick and was very opaque to the light to which cadmium sulfide is photosensitive. Such a structure has a photo-capacitive effect which apparently is in part the result of a heterojunction being formed between the cadmium sulfide and the cadmium telluride resulting from the decreased width of the depletion region which, in turn, is the result of the trapping at the junction of "holes" (positive charges) created by light excitation within the cadmium sulfide. See, for example, lines 50 to 59 of column 2 lines 19 through 30 of column 5 of the Beard patent.
It is to be noted that a uniform photoconductor films such as the cadmium sulfide used in the Beard patent always produces an exponential decrease in light intensity as a function of distance into the film which thus results in progressively less efficient light coupling for narrower depletion widths. Furthermore, any high purity, highly stoichiometric semiconductor film generally possesses a sharp spectral response which is required for low recombination velocities. In the case of cadmium sulfide, the spectral response is much too narrow for efficient coupling to a P-1 phosphor typical of commercial cathode ray tubes; and in fact, only a 10 percent efficiency results.
It is also to be noted that the incident light image is formed in the depletion width region and, the resolution of the device is determined by the distance from the depletion width region to the liquid crystal or other material being photocapacitively coupled to the photoconductor. Uniform films with broader spectral response generally possess higher deep trap densities and higher recombination center densities, which results in lower light injected carrier efficiencies.
Present Invention
Accordingly, one object of the present invention is to provide a photocapacitive light sensor of high sensitivity comprising a charge storage semiconductor diode.
A second object of the present invention is to provide such a photodiode having a broad spectral response with non-exponential carrier generation and low recombination velocities.
A third object of the present invention is to provide such a photodiode usable in high resolution display devices.
A fourth object of the present invention is to provide such a photodiode having high carrier injection efficiency.
A fifth object of the present invention is to provide such a photodiode usable in a high frequency alternating current device which is made possible by enhanced light-switching photocapacitive effects.
A broad object of the present invention is to provide such a photodiode structure which improves the performance of a high frequency alternating current display device.
The invention which satisfies these and other objects comprises a photodiode wherein the interface (in the particular exemplary embodiment described) between a film of cadmium sulfide photoconductor and a cadmium telluride light-blocking layer has adjacent to it a region of the photoconductor layer which has a higher density of defect centers than the rest of the photoconductor has. This region is characterized by a high incident light absorption coefficient and a high carrier injection coefficient. This high defect center density, high light absorption, high carrier injection region may be formed either by alloying cadmium sulfide with cadmium selenide near the interface, thus varying or grading the band gap across the cadmium sulfide film and introducing defect centers in the form of selenium atoms or by varying the defect center density of the cadmium sulfide itself across its thickness by progressively lowering the temperature during film preparation. Such a region may also be formed in other photoconductors/photocapacitors (such as ZnS, GaAs, and Si with photoresponse in different spectral regions) in accordance with the present invention.